The present invention relates to a resistor composition for resistor-incorporated spark plugs.
In the prior art, there is known a spark plug obtained by sealing a resistor of 0.5-20 K.OMEGA. in the centerbore of a refractory (ceramic) insulator between the inner end of an electrode rod (a center electrode) and the inner end of a terminal rod optionally by means of a conductive glass sealing agent, both rods being inserted through said centerbore, which is found to excel in the suppression of jamming radio waves generated by spark discharge.
The conventional compositions for such a spark plug generally comprise a base mixture composed of 30-70% by weight of glass frit with the remainder being an inorganic aggregate comprising alumina, zircon, mullite, silica, clay, silicon nitride, aluminium nitride or the like, or alternatively a mixture of two or more thereof, 0.1-10.0 parts by weight of a carbonaceous material (calculated on carbon after calcination) and 0-30 parts by weight of an agent for stabilizing resistance under load (hereinafter simply called the stabilizer) comprising one or more selected from the group consisting of the oxides and carbides of rare earth elements and metals of Subgroups IVa, Va and VIa of the periodic table as well as ZnO, B.sub.4 C, SiC, TiB and TiN.
It has been known to prepare such resistor compositions by mixing and calcining the glass and the inorganic aggregate with the so-called carbonaceous material producing carbon by calcination, and subsequently adding and mixing a binder to and with the resulting mixture. The stabilizer may be added before or after calcination. Known is also a process by which it is not necessary to calcine the glass, the inorganic aggregate and the carbonaceous substance prior to mixing in the binder and charging the mixture into the centerbore of the refractory insulator.
The resistor composition should be sealed in place with a given resistance value in order to prevent generation of any jamming of radio waves at the time of ignition of the spark plug. However, it has been found that both the aforesaid cases give rise to a variation in resistance value, and should rely upon strict sealing temperature control.